The present invention is directed to electrophotographic processes. More specifically, the present invention is directed to processes for maintaining the triboelectric stability of electrophotographic developers. In one embodiment, the present invention comprises an electrophotographic process wherein the triboelectric charging characteristics of the developer remain stable from initial use through an extended time period, with no need for mechanical preconditioning or subsequent monitoring and adjustment.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrophotographic imaging process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, entails placing a uniform electrostatic charge on a photoconductive insulating layer known as a photoconductor or photoreceptor, exposing the photoreceptor to a light and shadow image to dissipate the charge on the areas of the photoreceptor exposed to the light, and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic material known as toner. The toner will normally be attracted to those areas of the photoreceptor which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This developed image may then be transferred to a substrate such as paper. The transferred image may subsequently be permanently affixed to the substrate by heat, pressure, a combination of heat and pressure, or other suitable fixing means such as solvent or overcoating treatment.
Many methods are known for applying the electroscopic particles to the electrostatic latent image to be developed. One development method, disclosed in U.S. Pat. No. 2,618,552, is known as cascade development. Another technique for developing electrostatic images is the magnetic brush process, disclosed in U.S. Pat. No. 2,874,063. This method entails the carrying of a developer material containing toner and magnetic carrier particles by a magnet. The magnetic field of the magnet causes alignment of the magnetic carriers in a brushlike configuration, and this "magnetic brush" is brought into contact with the electrostatic image bearing surface of the photoreceptor. The toner particles are drawn from the brush to the electrostatic image by electrostatic attraction to the undischarged areas of the photoreceptor, and development of the image results. Other techniques, such as touchdown development, powder cloud development, and jumping development are known to be suitable for developing electrostatic latent images.
Often, when new developer is added to an electrophotographic imaging system, the triboelectric charging characteristics of the developer are unstable for a number of imaging cycles and reach a stable equilibrium only after a period of time. Typically, either the triboelectric charging value of the developer initially rises and then falls, finally reaching a point of equilibrium lower than the initial value, or the triboelectric charging value rises with time and reaches a point of equilibrium higher than the initial value. This initial instability usually results in disadvantages such as poor quality images for the first series of imaging cycles performed with the new developer, deterioration of image quality after a number of imaging cycles, the necessity for machine adjustment after the developer has been installed (incorporated) and has been in use for a period of time, or any combination of these problems. Accordingly, elimination of the initial instability of the triboelectric characteristics of new developers would reduce or eliminate the aforementioned difficulties. The process of the present invention overcomes this problem.
Attempts have been made to solve the problem of initial triboelectric instability of electrophotographic developers. One such solution entails mechanical preconditioning of the developer prior to use in order to simulate aging. The developer is agitated mechanically for a period of time such that the surfaces of the carrier particles become abraded. The process simulates the aging process that occurs through normal usage of the developer. Abrasion of the carrier surfaces is believed to be a contributing factor to the initial instability with respect to triboelectric characteristics often observed for new developers.
Mechanical preconditioning methods are disclosed in U.S. Pat. No. 3,970,571, which is directed to a method for preparing a preconditioned dry electrographic developer wherein the carrier particles are initially mixed with resin-based toner particles having a higher than usual concentration of a charge control agent, such as approximately 2 to 24 percent by weight. This initial mixing occurs by tumbling in a rotating container, and continues for a period sufficient to pack the carrier pores with toner particles, scum the carrier surfaces with the charge control agent, and abrade the carrier surfaces. After this process, resin-based toner particles having the charge control agent present in the amount desired in the final developer composition, such as approximately 1 percent by weight, are added to the developer. Thus, this process entails preconditioning of the developer for a period of time before its use in an imaging device.
Another potential solution to the problem of initial triboelectric instability resides in adjusting the development system after a number of imaging cycles, as disclosed in U.S. Pat. No. 4,288,518, which is directed to a method for addressing the problem of spent toner and eliminating the need to dispose of and replace developer after 10,000 to 20,000 imaging cycles. The method entails replenishing the developer composition with toner particles that, when mixed with the carrier of the initial developer composition, result in a developer differing in triboelectric charging characteristics from the initial developer. If the developer deterioration has resulted in images of increased image density, which is caused by a drop in triboelectric chargeability of the developer, the developer is replenished with new toner that will result in a developer having a lower chargeability than the initial developer. Conversely, if the developer deterioration has resulted in images of decreased image density, which is caused by a rise in triboelectric chargeability of the developer, the developer is replenished with new toner that will result in a developer having a higher chargeability than the initial developer. Replenishment according to this method may be delayed until after repeated use of the developer and replenishment with the original toner composition. This process requires adjustment of the development system after its initial installment by diagnosing a developer malfunction and replenishing the used developer with a toner having the necessary characteristics. In contrast, the present invention entails installing a fresh, unused developer package containing "start-up" toner in an imaging device and providing a replenishment package of "dispense" toner. This "dispense" toner is added to the developer gradually, as necessary to replenish the supply of toner in the development housing throughout the life of the developer, and no subsequent adjustments to the development system are necessary.
European Patent Application No. 248-119-A discloses a process wherein electrostatic images are developed using a developer comprising charged toner and carrier particles. During use, the toner particles are replenished with particles which have the same composition as the particles used initially, but have a larger mean particle diameter. The apparent density of the initial developer and the replenished developer does not differ by more than 10 percent.
Although the prior art processes are suitable for their intended purposes, they possess a number of disadvantages. For example, a mechanical preconditioning process constitutes an additional cost to the manufacturing process because of the required additional time and handling. In addition, mechanical preconditioning also abrades the carrier surfaces such that a large amount of carrier attrition occurs during the process. The small carrier pieces created by the abrasion can mix with the toner and ultimately become part of the developed image. These carrier pieces may adversely affect copy quality, especially when colored toners are used, since the pieces become trapped and fused with the toner and result in speckled images. Disadvantages also exist for a process involving the use of a developer until image quality deteriorates and subsequent replenishment of the used developer with a different toner selected according to the nature of the image deterioration, in that the process requiers monitoring of image quality during the lifetime of the developer and results in additional service costs. Accordingly, a need continues to exist for a process for eliminating initial triboelectric instability in developer compositions that requires no mechanical preconditioning or subsequent monitoring and maintenance.